mgcp_client_pool.h: add missing stdbool include

Fix for:
[   34s] In file included from vty.c:22:
[   34s] /usr/include/osmocom/mgcp_client/mgcp_client_pool.h:18:1: error: unknown type name 'bool'; did you mean '_Bool'?
[   34s]  bool mgcp_client_pool_member_is_blocked(const struct mgcp_client_pool_member *pool_member);

Related: https://obs.osmocom.org/package/live_build_log/osmocom:master/osmo-bsc-nat/CentOS_8/x86_64
Change-Id: Ic3773f49160cbb8c6f53c4f0e17b897017dec98a
diff --git a/include/osmocom/mgcp_client/mgcp_client_pool.h b/include/osmocom/mgcp_client/mgcp_client_pool.h
index 949e870..b547b78 100644
--- a/include/osmocom/mgcp_client/mgcp_client_pool.h
+++ b/include/osmocom/mgcp_client/mgcp_client_pool.h
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <stdbool.h>
+
 struct mgcp_client;
 struct mgcp_client_pool;
 struct mgcp_client_pool_member;